1. Field of the Invention
The present invention is related to an electro-optic sampling oscilloscope which measures a signal to be measured by employing optical pulses generated based on a signal from a delay circuit in which a trigger signal is delayed in response to the measured position of the waveform of the signal to be measured. More particularly, the present invention relates to an electro-optic sampling oscilloscope characterized in having a delay circuit in which a trigger signal is delayed.
This application is based on patent application No.Hei 9-273158 filed in Japan, the content of which is incorporated herein by reference.
2. Background Art
It is possible to observe the waveform of a signal to be measured by coupling the electric field generated by the signals to be measured to an electro-optic crystal, causing laser light to enter this electro-optic crystal, and using the polarization state of the laser light. Here, it is possible to use this laser light in pulse form, and to conduct measurement with extremely high time resolution when the sampling of the signal to be measured is conducted. An electro-optic sampling oscilloscope employs an electro-optic probe which takes advantage of this phenomenon.
In comparison with conventional sampling oscilloscopes which employ electrical probes, such an electro-optic sampling oscilloscope herein below termed an xe2x80x9cEOSxe2x80x9d oscilloscope) has the following characteristic features (Shinagawa, et al: xe2x80x9cA High-Impedance Probe Based on Electo-Optic Sampling,xe2x80x9d Proceeding of the 15th Meeting on Lightwave Sensing Technology, May 1995, pp 123-129):
(1) When signals are measured, a ground wire is not required, so that measurement is simplified.
(2) The metal pin which is at the lead end of the electro-optic probe is isolated from the circuit system, so that it is possible to realize a high input impedance, and as a result, the state at the point at which measurement is conducted is essentially free of fluctuations.
(3) Since optical pulses are employed, measurement is possible in a broad band up to the order of GHz.
The structure of an EOS oscilloscope will now be explained with reference to FIG. 4.
An EOS oscilloscope is composed of an EOS oscilloscope main body 1 and an electro-optic probe 2. In EOS oscilloscope main body 1, a trigger circuit 3 outputs a trigger signal which initiates measurement of the measurement signal. Delay circuit 4 then delays the signal from trigger circuit 3 by the time set by setting unit 9. Setting unit 9 is composed of switches and the like. The delay time set at setting unit 9 is set in delay circuit 4 via processing circuit 8. Timing generation circuit 5 generates the timing for generating the optical pulse and the timing for A/D conversion based on the signal from delay circuit 4. Optical pulse generating circuit 6 generates an optical pulse based on a timing signal from timing generation circuit 5.
The optical pulse from optical pulse outputting circuit 6 is supplied to electro-optic probe 2 and is subjected to polarization change by an electro-optic element. The optical pulse subjected to polarization change undergoes polarized light detection etc., by a polarized light detecting optical system (not shown) inside electro-optic probe 2, and that signal is input to EOS oscilloscope main body 1.
This signal is subjected to signal amplification or A/D conversion by A/D converter 7, and processing by processing circuit 8 for displaying the signal which is the target of measurement, an so on.
Next, FIG. 5 will be used to explain the reason for delaying the trigger signal from trigger circuit 3 using delay circuit 4. As shown in parts (a) and (b) in FIG. 5, the trigger signal from trigger circuit 3 is generated roughly in synchronization with the signal to be measured. In FIG. 5, the waveform from measurement point A1 or A2 is displayed on the display member (not shown) of EOS oscilloscope main body 1. The user of the EOS oscilloscope may not wish to display the waveform from measurement point A1 or A2, but rather may want to display the waveform from measurement point B1 or B2 which is slightly delayed from measurement point A1 or A2. In this way, delay circuit 4 is employed to shift the initial position of the display of the waveform.
For example, when the EOS oscilloscope user wants to display the waveform from measurement point B1 rather than measurement point A1, then setting unit 9 in FIG. 4 is set to delay the initial position of the display of the signal to be measured. The setting information is analyzed by processing circuit 8, delay time Txe2x80x2 for measurement point B1 with respect to measurement point A1 is determined, and a delay time Txe2x80x2 is set in delay circuit 4. As a result, as shown in part (c) in FIG. 5, delay circuit 4 outputs a trigger signal after delaying it by just delay time Txe2x80x2. Timing for sampling is then generated using the output from delay circuit 4.
FIG. 6 shows an example of one conventional structure for delay circuit 4. In FIG. 6, delay circuit 4 is composed of a lamp circuit 31 which designates the trigger signal as an input signal; a D/A converter 32 which converts the delay time setting signal from processing circuit 8 from a digital to an analog signal; and a comparing circuit 33 which compares the signal output from ramp circuit 31 and D/A converter 32.
The operation of the delay circuit shown in FIG. 6 will now be explained using FIG. 7.
As shown in part(a) in FIG. 7, when a trigger signal is input into ramp circuit 31, this signal is employed as a trigger for ramp circuit 31 to output a ramp signal as shown in part (c) in FIG. 7.
As shown in part (b) in FIG. 7, D/A converter 32 outputs an output signal in which the delay time setting signal from processing circuit 8 has been converted from a digital to an analog signal. When the value set from delay circuit 7 is delay time Txe2x80x2, then D/A converter 32 outputs a signal which is higher than the reference level by just a component corresponding to the delay time Txe2x80x2 only.
The outputs from ramp circuit 31 and D/A converter 32 are compared at comparing circuit 33, and a signal is output in which trigger signal has been delayed by just a time Txe2x80x2, as shown in part(c) in FIG. 7.
Depending on the measured signal, a relatively long delay may be desirable, on the order of milli-seconds or seconds, for example. In the above-described delay circuit 4 for an EOS oscilloscope, a ramp signal from ramp circuit 31 is employed to carry out the setting of the delay time. However, when carrying out a long delay time in this way, the ability to reproduce the signal is not good due to the generation of a shift or fluctuation (jitter) in the ramp signal. In other words, a delay time on the order of milli-seconds or seconds cannot be set with good accuracy.
Depending on the measured signal, a relatively long delay may be desirable, on the order of milli-seconds [ms] or seconds [s], for example. In the above-described delay circuit 4 for an EOS oscilloscope, a lamp signal from lamp circuit 31 is employed to carry out the setting of the delay time. However, when carrying out a long delay time in this way, the ability to reproduce the signal is not good due to the generation of a shift or fluctuation (jitter) in the lamp signal. In other words, a delay time on the order of milli-seconds [ms] or seconds [s] cannot be set with good accuracy.
The present invention was conceived in consideration of the above-described circumstances, and has as its objective the provision of an electro-optic sampling oscilloscope in which it is possible to set a stable delay time even in the case where a relatively long delay time is being set.
Therefore, the present invention provides an electro-optic sampling oscilloscope having a delay circuit which comprises a delay time detecting circuit, a regulation time determining circuit, a counter circuit and a delay regulating circuit. The delay time detecting circuit detects in the trigger signal a value corresponding to the delay time of a reference clock from a reference clock generating circuit. The regulation time determining circuit determines a regulation time based on the value detected by the delay time detecting circuit so that the regulation time is an integer multiple of the reference clock. The counter circuit is triggered by the trigger signal to count the reference clock through a specific value. The delay regulating circuit employs a signal related to the regulation time from the regulation time determining circuit, to delay the signal output from the counter circuit by the regulation time.
In this manner, it is possible to realize a delay which is an integer multiple of the period of the reference clock stably and with excellent accuracy even in the case where a relatively long delay time is being set.
Furthermore, the invention provides the electro-optic sampling oscilloscope having the delay circuit to which a micro-regulation value is enable to set.
In this manner, it is possible to set a delay time even more finely than period of the reference clock.
This summary of the invention dose not necessarily describes all necessary features so that the invention may also be a sub-combination of these described features.
The reference symbols used in the claims are not any influences for the interpretation of the claims. (only for EP application)